Synthesis of polyoxymethylene dimethyl ethers from methylal and trioxane catalyzed by Br?nsted acid ionic liquids with different alkyl groups

Article abstract of DOI:10.1039/c5ra08360e

Br?nsted acid ionic liquids with different alkyl group carbon chain lengths and an alkane sulfonic acid group were synthesized through bromoalkane, imidazole and 1,4-butane sultone as raw materials. The structures and properties of the ionic liquids were experimentally characterized. Catalytic reaction of methylal (DMM) with trioxane (TOX) for preparation of polyoxymethylene dimethyl ethers (PODME_n, CH₃O(CH₂O)_nCH₃, where n > 1) was investigated in various Br?nsted acid ionic liquids with different carbon chain length of alkyl groups. The carbon chain length of alkyl groups and activity correlation for the ionic liquids was studied. It was found that the structures of ionic liquids were consistent with the designed structure and their purities were high. They possessed high thermal stability and wide liquid range. The hydrophobicity of ionic liquids became stronger with the increase of carbon chain length. With increasing the carbon chain length of ionic liquids, the selectivity of PODME_3-8 is increased at first and then decreased. Among all the ionic liquids, [C₆ImBS][HSO₄] shows the best catalytic performance and the selectivity of PODME_3-8 is 57.85%.

Full text of DOI:10.1039/c5ra08360e

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PAPER
Synthesis of polyoxymethylene dimethyl ethers
from methylal and trioxane catalyzed by Brønsted
acid ionic liquids with different alkyl groups
Cite this: RSC Adv., 2015, 5, 57968
Qin Wu, Weijiao Li, Min Wang, Yu Hao, Tonghua Chu, Jiqing Shang, Hansheng Li,*
Yun Zhao and Qingze Jiao
Brønsted acid ionic liquids with different alkyl group carbon chain lengths and an alkane sulfonic acid group
were synthesized through bromoalkane, imidazole and 1,4-butane sultone as raw materials. The structures
and properties of the ionic liquids were experimentally characterized. Catalytic reaction of methylal (DMM)
n 3 2 n 3
with trioxane (TOX) for preparation of polyoxymethylene dimethyl ethers (PODME , CH O(CH O) CH ,
where n > 1) was investigated in various Brønsted acid ionic liquids with different carbon chain length of
alkyl groups. The carbon chain length of alkyl groups and activity correlation for the ionic liquids was
studied. It was found that the structures of ionic liquids were consistent with the designed structure and
their purities were high. They possessed high thermal stability and wide liquid range. The hydrophobicity
of ionic liquids became stronger with the increase of carbon chain length. With increasing the carbon
chain length of ionic liquids, the selectivity of PODME3–8 is increased at first and then decreased. Among
Received 6th May 2015
Accepted 26th June 2015
DOI: 10.1039/c5ra08360e
all the ionic liquids, [C
6
ImBS][HSO
4
] shows the best catalytic performance and the selectivity of
www.rsc.org/advances
PODME3–8 is 57.85%.
PODMEn is usually produced by the end-group (–CH3)
Introduction
provider and the chain-group (–CH O–) provider with the
2
Diesel engines have revolutionized transportation due to their existence of catalysts. Generally, the former contains meth-
high thermal efficiency. Environmental and linked regulatory anol, methylal or dimethyl ether, the latter contains trioxane,
5
–7
issues have driven the development of low emission diesel formaldehyde or paraformaldehyde.
engines. Still, even in modern high performance diesel engines, CF SO H as the catalyst to prepare PODME
the formation of soot during the combustion remains a and paraformaldehyde as starting materials. However,
Patrini et al. used
3
3
n
, with methylal
9
1
0
problem. Oxygenated compounds (molecules which contain homogeneous liquid catalysts, such as sulfuric acid,
oxygen in their structure) are known to reduce soot formation CF SO H, are corrosive and are not easily recovered, leading
during the combustion, when added to diesel fuels. Polyoxy- to the release of environmentally unfriendly effluents, which
methylene dimethyl ethers (PODME
) are novel oxygenated inevitably leads to a series of environmental problems.
compounds with the formula CH O(CH O) CH
, where n $ 1. Arvidson et al. prepared PODME by the reaction of trioxane,
The features of PODME are high oxygen content (42–51%) and methylal and paraformaldehyde using cation exchange resin
3
3
1,2
n
3
2
n
3
n
n
1
1
high cetane number, which can improve the combustibility of as catalyst. Although heterogeneous acid catalysts are
diesel oil, enhance the efficiency of combustion, and reduce environmentally friendly, they also have some drawbacks,
Thus, it is believed to have wide application such as low catalytic activity and easy deactivation of the
prospect as a diesel additive. However, considering the inu- catalyst. Therefore, the development of environmental and
1
2
3–7
pollutants.
4
ences of vapor pressure, boiling point, and solubility of efficient catalysts for synthesis of PODME
is extremely
n
PODME
n
, the suitable n value is 3–8, and the average cetane urgent.
8
number of PODME3–8 is up to 76. It was reported that the
In recent years, room temperature ionic liquids, as envi-
emissions of particulate pollutants and NO can be reduced up ronmentally benign and “designer” solvents or catalysts, have
x
to 80–90% and 50%, respectively, by adding 20% PODME to attracted much attention due to numerous advantages, such as
3
–8
5
diesel oil.
wide liquid range, high catalytic activity, ignorable vapor
pressure, designed structure and easy separation from the
13–20
reaction system.
Chen et al. prepared PODMEn from
Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of
Chemical Engineering and the Environment, Beijing Institute of Technology, 100081,
Beijing, China. E-mail: hanshengli@bit.edu.cn; Fax: +86 10 68918979; Tel: +86 10
methanol and trioxane by employing ionic liquid as the cata-
lyst. But the yield of PODME3–8 was low for the reaction of
methanol and trioxane. In our previous work, several Brønsted
2
1
6
8918979
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acid ionic liquids with different nitrogen groups and different
alkane sulfonic acid groups were synthesized. Catalytic reac-
tion of methylal with trioxane for preparation of polyoxy-
methylene dimethyl ethers was investigated in these ionic
liquids. The Brønsted acidity–activity correlation for the ionic
2
2
liquids was studied.
In this work, Brønsted acid ionic liquids with different
carbon chain length of alkyl groups were synthesized. They were
characterized by infrared spectrum (IR), nuclear magnetic
resonance (NMR), electrospray ionization mass spectrum (ESI-
MS), solubility analysis and differential thermal analysis (TG-
DTA). Their catalytic performances for the synthesis of
PODME
n
from methylal (DMM) and trioxane (TOX) were
investigated in various Brønsted acid ionic liquids with
different carbon chain length of alkyl groups. The carbon chain
length of alkyl groups and activity correlation for the ionic
liquids was studied.
Experimental
Materials
Sodium, bromomethane, bromoethane, 1-bromobutane, 1-
bromohexane and 1-bromooctane were purchased from Sion-
pharm Chemical Reagent Co., Ltd. 1-Bromodecane, 1-bromo-
dodecane, 1-bromotetradecane, 1-bromohexadecane, methylal
and trioxane were obtained from Alfa Aesa. 1,4-Butane sultone
were purchased from WuHan Fengfan Chemical Factory.
Imidazole was obtained from Linhai Kaile Chemical Factory.
Ethanol, acetone, dichloromethane, ether, ethyl acetate and
sulfuric acid were received from Beijing Chemical Co., Ltd. All
reagents and starting materials were commercially available
and used without any further purication unless, otherwise,
noted.
Scheme 1 Synthetic procedure of Brønsted acid ionic liquids with
different alkyl groups.
(1) Preparation of alkylimidazole with different carbon
chain (RIm). The sodium was slowly added to ethanol. The
produced sodium ethoxide solution, which was colorless
transparent liquid, was puried through ltration.
Methods
The imidazole was added to sodium ethoxide solution, and
The structures of the ionic liquids and their intermediates were the mixture was stirred in the oil bath pan for eight hours,
analyzed by IR spectroscopy, NMR and ESI-MS. IR spectra were resulting in the formation of the sodium imidazole. The sodium
recorded on a Perkin-Elmer Spectrum One FTIR spectrometer imidazole was puried through ltration.
using KBr windows suitable for Fourier transform infrared
A stoichiometric amount of RBr was added to the ethanol
FTIR) transmittance technology to form a liquid lm. H NMR solution of sodium imidazole, and the mixture was stirred in
1
(
spectra were obtained on a Varian mercury-plus 400 MHz the oil bath pan for two days, resulting in the formation of the
nuclear magnetic resonance spectrometer. Chemical shis were alkylimidazole (RIm). The RIm was washed repeatedly with
reported in parts per million (ppm, d). ESI-MS spectra were ethanol, acetone and dichloromethane, leached and distilled
obtained on Varian 500-MS instrument. The thermal decom- accordingly. Finally, the RIm was puried by column chroma-
position temperature was determined by Perkin-Elmer TGA7 tography on silica gel.
ꢀ
ꢁ1
TG-DTA analysis, with a heating rate of 10 C min from room
4
(2) Preparation of acidic ionic liquids ([RImBs][HSO ]). A
ꢀ
temperature to 700 C. The melting points of the ionic liquids stoichiometric amount of 1,4-butane sultone was added drop-
with different carbon chain of alkyl groups were measured by wise to the RIm at 0 C and the mixture was stirred at 50 C for
TA DSC Q2000. The viscosity of the ionic liquids with different 2–3 days. The zwitterions BsImR produced, all of which were
ꢀ
ꢀ
carbon chain of alkyl groups was measured by capillary vis- white solids, were washed three times with ethyl acetate and
ꢀ
cosimetry at 60 C.
ether to remove unreacted material and dried for 12 hours in a
vacuum.
Preparation of Brønsted acid ionic liquids with different alkyl
groups
A stoichiometric amount of concentrated sulfuric acid was
ꢀ
added dropwise to the zwitterions at 0 C, and the mixture was
ꢀ
stirred at 50 C for 1–2 days, resulting in the formation of the
The synthetic procedure of Brønsted acid ionic liquids with
different alkyl groups is illustrated in Scheme 1.
[
RImBs][HSO
4
] ionic liquids. The ionic liquids were washed
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repeatedly with ethyl acetate and ether to remove unreacted
23
material and dried under vacuum.
Solubility measurement of the [RImBs][HSO
4
] ionic liquids
The method for the solubility measurement of ionic liquids in
selected solvents was the same as that described in the litera-
24
ture. The equilibrium cell was a sealed 120 mL glass
measuring ask. The ask was immersed in a constant-
temperature water bath. A magnetic stirrer was utilized for
solution preparation. The precision of the analytical balance
was 0.1 mg.
Synthesis of polyoxymethylene dimethyl ethers (PODMEn)
catalyzed by the [RImBs][HSO ] ionic liquid
4
The reaction of methylal with trioxane was carried out in a 50
mL Teon-lined stainless-steel autoclave, equipped with ther-
mostat and mechanical stirring. Methylal, trioxane, and
8 8
Fig. 1 IR spectra of C H17Im and C H17Br.
[
4
RImBs][HSO ] ionic liquid catalyst with different molar rations
[
R ImBS][HSO ] ILs, the peak intensity of sulfonic acid group
8 4
were quantitatively introduced into the reactor successively. The
reaction was allowed to proceed for 10 h and kept at the desired
temperature. Aer the reaction completed, the mixture was
cooled and kept at 0 C for 0.5 h and two phases were formed.
The upper phase consisted of the produced PODME and the
unreacted materials, and the lower phase contained the ionic
liquid catalyst. The lower phase was simply separated from the
upper phase by decantation.
The upper solution was analyzed by a Shimadzu GC-2014
equipped with a FID detector. A capillary column (DB-5MS, 30
m ꢂ 0.25 mm, id ꢂ 0.25 mm) was used. The quantitative analysis
was carried out by internal standard method. The internal
standard compound was methanol. The calculation of trioxane
conversion and PODME selectivity was based on carbon
balance. Analytical conditions were as follows: injection port
increased obviously compared with the R
cm band was assigned to the –SO
indicative of the existence of main group. In addition, the
wide peak at 3300–3500 cm was assigned to the O–H group
8
ImBS. The 596
H group vibration further
ꢁ1
4
ꢀ
ꢁ1
n
of water because the [R
absorbable. The similar results were obtained for the other
R ImBS][HSO ] ionic liquids. Therefore, the IR data are
8 4
ImBS][HSO ] is super water-
[
n
4
consistent with the structures for the ionic liquids shown in
Scheme 1.
NMR analysis of the [RImBS][HSO ] ionic liquids
4
1
The ionic liquids were analyzed by H NMR spectroscopy. The
NMR spectral data of the ionic liquids are as follows.
1
C
1
-IL([C
1
ImBS][HSO
4
]): H NMR (400 MHz, DMSO-d
6
): d
ꢀ
ꢀ
temperature, 290 C; FID temperature, 300 C; oven tempera-
ture program, initially the temperature was held at 40 C for 5
1
2
.540 (m, 2H), 1.877 (m, 2H), 2.496 (t, 2H), 3.852 (s, 3H), 4.183 (t,
ꢀ
H), 7.707 (s, 1H), 7.772 (s, 1H), 9.137 (s, 1H).
ꢀ
ꢀ
ꢁ1
min, then ramped to 280 C at a rate of 5 C min and held for
min; carrier gas, nitrogen of 0.4 Mpa; reagent gases, air of 0.4
1
C
2
-IL([C
2
ImBS][HSO
4
]): H NMR (400 MHz, DMSO-d
6
): d
5
1
(
.420 (m, 3H), 1.547 (m, 2H), 1.871 (m, 2H), 2.510 (t, 2H), 4.201
m, 4H), 7.807 (s, 1H), 7.833 (s, 1H), 9.250 (s, 1H).
Mpa, hydrogen of 0.3 Mpa.
Results and discussion
IR analysis of the intermediate [RIm] and [RImBs][HSO4]
8 8
Fig. 1 show the IR spectra of C H17Im and C H17Br. For the
ꢁ
1
ꢁ1
C H Im, the bands at 3130 cm and 1511 cm were assigned
8
17
to the C–H and C–N ring stretching vibration of imidazole. In
ꢁ
1
addition, the 550 cm band peak disappeared or weakened. It
was shown that the imidazole group had substituted Bromine
group and the C
8
H17Im had been synthesized successfully.
Similarly, the C
respectively.
n
H2n+1Im had been synthesized successfully,
Fig. 2 display the infrared spectra of the R
8 8
Im, R ImBS and
[
R ImBS][HSO ] ionic liquids. As can be seen from Fig. 2, for
8
4
ꢁ
1
the BsImR , the bands at 1245 and 1167 cm were assigned
to the O]S]O stretching vibrations. The 1040 cm band
was assigned to the C–S–O stretching vibrations. These
8
ꢁ
1
8 8 8 4
results showed the existence of sulfonic acid group. For the Fig. 2 IR spectra of R Im, R ImBS and [R ImBS][HSO ].
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1
C
4
-IL([C
4
ImBS][HSO
4
]): H NMR (400 MHz, DMSO-d
6
): d IL and C16-IL were 311, 301, 288, 274, 273, 280, 296, 265 and
ꢀ
0
1
.9160 (m, 3H), 1.276 (m, 2H), 1.536 (m, 2H), 1.776 (m, 2H), 253 C, respectively. It was shown that these ionic liquids
.886 (m, 2H), 2.506 (t, 2H), 4.187 (m, 4H), 7.815 (s, 2H), 9.237 possessed high thermal stability and wide liquid range, which
ꢀ
(s, 1H).
6 4 6
-IL([C ImBS][HSO ]): H NMR (400 MHz, DMSO-d ): d
are more than 250 C.
1
C
6
0
1
.8020 (m, 3H), 1.210 (m, 6H), 1.510 (m, 2H), 1.738 (m, 2H),
.848 (m, 2H), 2.529 (t, 2H), 4.139 (m, 4H), 7.775 (s, 2H), 9.228
Melting points of the ionic liquids
The melting points of the ionic liquids with different carbon
chain of alkyl groups were measured by TA DSC Q2000. The
melting points of ionic liquids C -IL, C -IL, C -IL, C -IL, C -IL,
1 2 4 6 8
(s, 1H).
8 4 6
-IL([C ImBS][HSO ]): H NMR (400 MHz, DMSO-d ): d
1
C
8
0
1
.9120 (m, 3H), 1.250 (m, 10H), 1.521 (m, 2H), 1.752 (m, 2H),
.872 (m, 2H), 2.509 (t, 2H), 4.187 (m, 4H), 7.812 (s, 2H), 9.227
C
ꢁ
10-IL, C12-IL, C14-IL and C16-IL were <ꢁ80, <ꢁ80, <ꢁ80, ꢁ68,
ꢀ
51, ꢁ41, ꢁ5, 55 and 63 C, respectively. It was shown that
(s, 1H).
4 6
10-IL([C10ImBS][HSO ]): H NMR (400 MHz, DMSO-d ): d
these ionic liquids possessed wide liquid range.
1
C
0
1
.8610 (m, 3H), 1.282 (m, 14H), 1.532 (m, 2H), 1.781 (m, 2H),
.852 (m, 2H), 2.521 (t, 2H), 4.334 (m, 4H), 7.781 (s, 2H), 9.208
Viscosity analysis of the ionic liquids
(
s, 1H). The viscosity of the ionic liquids with different carbon chain of
4 6
12-IL([C12ImBS][HSO ]): H NMR (400 MHz, DMSO-d ): d alkyl groups was measured by capillary viscosimetry. Fig. 4
1
C
0
.8112 (m, 3H), 1.253 (m, 10H), 1.521–1.853 (m, 10H), 2.532 displays the viscosity of the different ionic liquids. As could be
seen from Fig. 4, when the carbon chain length of ionic liquids
C -IL([C ImBS][HSO ]): H NMR (400 MHz, DMSO-d ): d is less than 6, the viscosity of ionic liquid has a slight decrease.
(t, 4H),4.139 (m, 6H), 7.775 (s, 2H), 9.205 (s, 1H).
1
1
4
14
4
6
0.8921 (m, 3H), 1.117 (m, 20H), 1.511–1.832 (m, 6H), 2.532 However, when the carbon chain length of ionic liquids
(
t, 4H), 4.176 (m, 4H), 7.871 (s, 2H), 9.204 (s, 1H).
increases from 6 to 10, the viscosity of ionic liquid sharply
): d increases. It was shown that the carbon chain length of ionic
1
C
16-IL([C16ImBS][HSO
4
]): H NMR (400 MHz, DMSO-d
6
0.9612 (m, 3H), 1.031 (m, 22H), 1.511–1.801 (m, 6H), 2.521 liquids has important effect on the viscosity of the ionic
(t, 6H), 4.210 (m, 4H), 7.775 (s, 2H), 9.119 (s, 1H).
liquids.
The NMR spectral data of the ionic liquids agreed with their
designed structures (Scheme 1). As can be seen from the spec-
trum of these ionic liquids, there was no impurity peak in the H
NMR spectrum. This demonstrated that the purity of the ionic
liquids was high. Therefore, the synthesis and purication
methods for the ionic liquids were reliable.
4
Solubility analysis of the [RImBS][HSO ] ionic liquids
1
The solubility of ionic liquids in methanol was studied at
different temperature (see Table 1). As shown in Table 1, the
[
C
1–8ImBS][HSO
4
] ionic liquids were miscible in methanol. For
] ionic liquids, the solubility of ionic
the [C10–16ImBS][HSO
4
liquids decreased with the increase of carbon chain length at
each selected temperature and increased with the increase of
4
ESI-MS analysis of the [RImBS][HSO ] ionic liquids
The positive and negative ion electrospray ionization mass temperature. It was shown that the hydrophobicity of ionic
spectra of the [C ImBS][HSO ] ionic liquid are shown in Fig. 3 liquids became stronger with the increase of carbon chain
the upper spectra are the positive ion spectra and the lower length.
spectra are the negative ion spectra). As seen from Fig. 3, large The solubility of ionic liquids in methylal was studied at
8
4
(
peak in the positive ion mode occurred at m/z 317.1, corre- different temperature (see Table 2). As shown in Table 2, the
sponding to the positive ion of [C ImBS][HSO ]. In addition, the ionic liquids were slightly soluble in methylal.
8
4
negative ion mode showed the presence of peak at m/z 96.9,
corresponding to the HSO4 ion. The peak in the negative ion rated from methylal via simple decantation and reused.
It was seen that the ionic liquid catalyst can be easily sepa-
ꢁ
mode at m/z 315.2 may be assigned as a negative ion being
+
produced by the loss of two H from the positive ion of ionic
Reaction of methylal with trioxane for preparation of PODMEn
liquid. The similar results were obtained for the other [C
n
ImBS]-
The effect of ionic liquids with different carbon chain of alkyl
groups on the catalytic reaction was studied (see Fig. 5). Fig. 5(a)
displays that the conversions of trioxane (XTOX) are all above
[
HSO ] ionic liquids. Large peaks in the positive ion mode
4
occurred at m/z 219, 233, 261, 289, 317, 345, 373, 401 and 429,
corresponding to the positive ion of ionic liquids (n ¼ 1–14)
respectively. All negative ion modes showed the presence of peak
90% for the different carbon chain of ionic liquids, but the
selectivity of the PODME3–8 are distinctly different. As the
carbon chain length of ionic liquids increased, the conversion
of trioxane, which exceeds 90%, is basically unchanged. When
the carbon chain length of ionic liquids is less than 6, the
selectivity of PODME3–8 has a slight increase. However, when
the carbon chain length of ionic liquids increases from 6 to 16,
ꢁ
at m/z 97, corresponding to the HSO4 ion. Therefore, the ESI-
MS data are consistent with the structures for the ionic liquids
shown in Scheme 1.
4
TG-DTA analysis of the [RImBS][HSO ] ionic liquids
Thermal decomposition temperature was determined by TG- the selectivity of PODME3–8 sharply decreases. The carbon chain
DTA analysis. The thermal decomposition temperatures of length of ionic liquid has a signicant effect on its catalytic
ionic liquids C
1
-IL, C
2
-IL, C
4
-IL, C
6
-IL, C
8
-IL, C10-IL, C12-IL, C14
-
6 4
activity. Among all the ionic liquids, [C ImBS][HSO ] shows the
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8 4
Fig. 3 Electrospray ionization mass spectra (ESI-MS) of the [C ImBS]HSO .
Table 1 Solubility (mass fraction) of the ionic liquids in methanol at
different temperatures
ꢀ
ꢀ
ꢀ
ꢀ
Ionic liquids
5 C
20 C
30 C
40 C
[C
[C
[C
[C
[C
[C
[C
[C
[C
1
2
4
6
8
ImBS]HSO
ImBS]HSO
ImBS]HSO
ImBS]HSO
ImBS]HSO
10ImBS]HSO
12ImBS]HSO
14ImBS]HSO
16ImBS]HSO
4
4
4
4
4
Miscible
Miscible
Miscible
Miscible
Miscible
63.74
58.35
41.84
31.36
Miscible
Miscible
Miscible
Miscible
Miscible
82.56
73.75
52.35
48.94
Miscible
Miscible
Miscible
Miscible
Miscible
Miscible
85.73
Miscible
Miscible
Miscible
Miscible
Miscible
Miscible
Miscible
86.75
4
4
4
4
64.68
57.90
67.94
Fig. 4 The viscosity of the ionic liquids with different carbon chain of activity. In addition, the catalytic activity of the ionic liquids is
alkyl groups.
related to the viscosity of the ionic liquids. When the viscosity of
the ionic liquids is high, the collision rate between the ionic
Table 2 Solubility (mass fraction) of the ionic liquids in methylal at
best catalytic performance and the selectivity of PODME3–8 is
different temperatures
5
7.85%. On the basis of the above discussion of the hydro-
phobicity of the ionic liquids (see the section of solubility
analysis of the [RImBS][HSO ] ionic liquids), the catalytic
ꢀ
ꢀ
ꢀ
Ionic liquids
10 C
20 C
40 C
4
[
[
[
[
C
C
C
C
1
4
6
8
ImBS][HSO
ImBS][HSO
ImBS][HSO
ImBS][HSO
4
4
4
4
]
]
]
]
0.25
0.11
0.19
0.23
0.19
0.25
0.21
0.29
0.17
0.14
0.13
0.23
0.26
0.31
0.25
0.19
0.25
0.30
activity of the ionic liquid may be related to the hydrophobicity.
A proper hydrophobicity contributes to the catalytic reaction of
methylal with trioxane for preparation of PODME . However,
n
when the carbon chain length of ionic liquids is greater than 6, [C10ImBS][HSO4]
the effect of steric hindrance on reaction reduces catalytic [C12ImBS][HSO
4
]
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than 6, the viscosity of ionic liquid has a slight decrease.
However, when the carbon chain length of ionic liquids
increases from 6 to 10, the viscosity of ionic liquid sharply
increases. The viscosity of [R ImBS][HSO ] ionic liquids is the
6
4
lowest among all the ionic liquids. Therefore, the carbon chain
length of ionic liquids have effect on the catalytic activity of the
ionic liquids. Among all the ionic liquids, [C ImBS][HSO ]
6 4
shows the best catalytic performance and the selectivity of
PODME3–8 is 57.85%. With increasing the carbon chain length
of ionic liquids, the variation trend of viscosity is contrary to
that of catalytic activity.
In addition, as shown in Fig. 5(b), with increasing the
carbon chain length of ionic liquids, the selectivity of
PODME
rst and then decreased and the selectivity of PODME4,5
is slightly increased. The selectivity of the PODME
2 3
, PODME , and PODMEn¼6–10 are increased at

n
decreases with the increase of polymerization degrees.
The selectivity of PODMEn is in the sequence of n ¼ 2 >
n ¼ 3 > n ¼ 4 > n ¼ 5 > n ¼ 6 > n ¼ 7 > n ¼ 8. Therefore,
the reaction mechanism is reliable in Fig. 6. First trioxane
is converted to formaldehyde over the acidic catalyst.
Formaldehyde represents the monomer of the PODME olig-
omers and reacts with methylal (DMM) to PODMEn¼2. In
n
general PODME may react with formaldehyde to obtain
2
PODMEn+1
.
The reuse of ionic liquid
In order to investigate the potential reusability of the ionic
liquids in the synthesis of PODME , a series of recycle
n
experiments were conducted with a methylal to trioxane
molar ratio of 3 and methylal to ionic liquid molar ratio of 180
at 150 C for 10 h. Aer each recycle reaction, the ionic liquid
was simply separated from the upper phase by decantation
and was dried for 12 h in vacuum. As shown in Fig. 7, the
Fig. 5 Effect of ionic liquids with different carbon chain of alkyl groups
on the catalytic reaction. Reaction conditions: n (methylal) : n (tri-
oxane) : n (ILs) ¼ 180 : 60 : 1, 150 C, 10 h.
ꢀ
ꢀ
6 4
[C ImBS][HSO ] was used ve times and its catalytic activity
changed very little. The recovery rate of the ionic liquid
catalyst was 91% aer used ve times. Therefore, the
[C ImBS][HSO ] as the catalyst for the synthesis of PODME is
6 4 n
recyclable and stable.
liquid catalyst and reactants becomes small. So, the reaction
rate is slow. On the contrary, when the viscosity of the ionic
liquids is low, the reaction rate is fast. On the basis of the above
discussion of the viscosity of the ionic liquids (see the section of
viscosity analysis of the ionic liquids), the carbon chain length
of ionic liquids has important effect on the viscosity of the ionic
liquids. When the carbon chain length of ionic liquids is less
Fig. 6 Reaction mechanism of PODME
n
.
Fig. 7 The reuse of ionic liquid.
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RSC Adv., 2015, 5, 57968–57974 | 57973

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RSC Advances
Paper
4
Y. Lei, Q. Sun, Z. Chen and J. Shen, Acta Chim. Sin., 2009, 67,
Conclusions
767–772.
Brønsted acid ionic liquids with different carbon chain length
of alkyl groups and an alkane sulfonic acid group were
synthesized through bromoalkane, imidazole and 1,4-butane
sultone as raw materials. They were characterized by infrared
spectrum (IR), nuclear magnetic resonance (NMR), ESI-MS,
solubility analysis and differential thermal analysis (TG/DTA).
Catalytic reaction of methylal (DMM) with trioxane (TOX) for
preparation of polyoxymethylene dimethyl ethers (PODMEn,
5 Q. Zhao, H. Wang, Z. F. Qin, Z. W. Wu, J. B. Wu, W. B. Fan
and J. G. Wang, J. Fuel Chem. Technol., 2011, 39, 918–923.
6 J. Burger, E. Strofer and H. Hasse, Ind. Eng. Chem. Res., 2012,
51, 12751–12761.
7 L. Pellegrini, M. Marchionna, R. Patrini and S. Florio, SAE
Tech. Pap., 2013, DOI: 10.4271/2013-01-1035.
8 K. J. Liu, C. F. Zhang and R. F. Li, Chem. Ind. Eng. Prog., 2013,
32, 2593–2598.
CH
3
O(CH
2
O)
n
CH
3
, where n > 1) was investigated in various
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Brønsted acid ionic liquids with different carbon chain length 10 E. Stroefer, H. Schelling and H. Hasse, Patent EP., 1902009,
of alkyl groups. It was found that their structures were consis-
2008.
tent with the designed structure and their purity was high. The 11 M. Arvidson, M. E. Fakley and M. S. Spencer, J. Mol. Catal.,
thermal decomposition temperatures of ionic liquids were
1987, 47, 391–393.
ꢀ
more than 250 C by analyzing TG/DTA. The hydrophobicity of 12 Y. Y. Zheng, Q. Tang, T. F. Wang, Y. H. Liao and J. F. Wang,
ionic liquids became stronger with the increase of carbon chain
length. These acidic ionic liquids showed excellent catalytic 13 L. Y. Piao, X. Fu, Y. L. Yang, G. H. Tao and Y. Kou, Catal.
performance. The catalytic activity of each ILs is dependent on
Today, 2004, 93, 301–305.
the carbon chain length of alkyl groups. The selectivity of 14 Y. L. Gu, F. Shi and Y. Q. Deng, J. Mol. Catal. A: Chem., 2004,
PODME3–8 are increased at rst and then decreased with
212, 71–75.
lengthening carbon chain of ionic liquids. The ionic liquid – 15 D. C. Forbes and K. J. Weaver, J. Mol. Catal. A: Chem., 2004,
Chem. Eng. Technol., 2013, 36, 1951–1956.
[C
6
ImBS][HSO
4
] showed the best catalytic performance.
214, 129–132.
1
1
6 A. K. Ressmann, P. Gaertner and K. Bica, Green Chem., 2011,
13, 1442–1447.
Acknowledgements
7 Q. Wu, H. Chen, M. H. Han, D. Z. Wang and J. F. Wang, Ind.
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This work is supported by the National Nature Science Foun-
dation of China (20706006), the Excellent Young Scholars 18 M. H. Han, W. L. Yi, Q. Wu, Y. Liu, Y. C. Hong and
Research Fund of Beijing Institute of Technology (2007Y0509)
D. Z. Wang, Bioresour. Technol., 2009, 100, 2308–2310.
and the Fundamental Research Foundation of Beijing Institute 19 Q. Wu, H. L. Wan, H. S. Li, H. R. Song and T. H. Chu, Catal.
of Technology (20121042007) and the Foundation of Beijing Key
Today, 2013, 200, 74–79.
Laboratory for Chemical Power Source and Green Catalysis 20 L. B. Ding, H. S. Li, Y. P. Zhang, H. Yuan, Q. Wu, Y. Zhao,
(2013CX02031, 2014CX02026).
Q. Z. Jiao and D. X. Shi, RSC Adv., 2015, 5, 21415–21421.
2
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57974 | RSC Adv., 2015, 5, 57968–57974
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